The present invention relates to forward converters. More particularly, the invention relates to forward converters having a storage inductance, a clock switch connected in series with a winding of the storage inductance and actuated by a control circuit as a function of the magnitude of the output voltage relative to a set reference voltage, and a MOS field effect transistor (MOSFET) which is operated inversely to the clock switch and, during the ON periods of the clock switch, permits the establishment of a free running circuit via the storage inductance and the connection of a load to the output of the forward converter.
A forward converter of this type is disclosed, for example, in DE-OS 3,727,170. Here (see, for example, FIG. 2 of the DE-OS) a forward converter equipped with a transformer former employs, instead of the otherwise customary power diodes on the secondary side, power MOSFET's which in the ON state exhibit a lower voltage drop across their source-drain path than diodes or Schottky diodes and thus generate lower losses than such diodes or Schottky diodes.
In the prior art forward converter, the signal for actuating the MOSFET's is obtained from special secondary windings of the transformer and is fed to the MOSFET gates by way of complicated dual-terminal structures. In addition to the costs for the additional transformer windings and the dual-terminal components, this has the drawback that the switching times of the MOSFET's are shifted relative to those of the clock switch on the primary side due to the switching delay caused by the transformer inductance. Thus accurate operation in synchronism with the clock switch so as to guarantee optimum efficiency is not ensured.
If a demagnetization coil is employed there exists the danger, particularly if the primary voltage source is overloaded, that the necessary control voltage for the inversely operated MOSFET at the output of the secondary winding furnishing the control voltage for the MOSFET is realized too late or not at all.
In a forward converter operated without transformer, actuation by way of secondary windings as in the prior art is impossible in any case.